It is known that an optical signal is attenuated during transmission through an optical fiber. The optical signals, therefore, have to be amplified. Two physical criteria are critical for the optical transmission of signals: firstly the gain and secondly the optical signal-to-noise ratio (OSNR). In the case of WDM (wavelength division multiplex) systems, these two criteria must be adequate and constant for all WDM signals over a wide range of frequencies. Ideally, the signal level and signal-to-noise ratio curves will be flat. The use of distributed Raman amplification in the transmission fiber allows the characteristics of optical transmission systems to be considerably improved. All the WDM channels (discrete signals) at the end of the fiber must exhibit at least approximately equal amplitudes. If the gain spectra were not flat, the dynamic range of the receiver would be exceeded or not completely utilized.
The purpose of optimization is normally to provide a maximally flat gain spectrum or maximally equal output levels of the individual channels at the end of the transmission fiber section. A very flat gain spectrum over a wide wavelength range can only be achieved by using a large number of pump wavelengths. To date, known distributed Raman amplifiers for WDM transmission systems employ, for example, several pump lasers whose output signals each exhibit a different wavelength.
“Low noise operation of Er3+ doped silica fibre amplifier around 1.6 μm”, J. F. Massicott, R. Wyatt, B. J. Ainslie, Electronics Letters, Vol. 28, No. 20, 24th Sep. 1992 describes an amplifier for a signal bandwidth of 40 nm between 1.57 and 1.61 μm. Here pump signals from two pump lasers are injected into the fiber. The wavelength of the signal to be transmitted is 1.6 μm and the wavelength of the first pump laser is 1.48 μm. The larger 1.55 μm wavelength and the output power of the second pump laser are selected such that the WDM channels distributed over a bandwidth of 40 nm are amplified as uniformly as possible at 35% population inversion of the amplifier at the WDM signal input. In the reference quoted, different pump source output powers are tested to optimize gain and OSNR. As shown in FIG. 2 of this reference, the gain and noise power curves are optimized over a bandwidth of approximately 40 nm for a selected pump power of 87 mW at 1.55 μm and an additional pump power of −17 dBm at 1.48 μm. The noise power variations are approximately 1 dB for a gain of 24 dB. Higher gains up to 31 dB can be achieved if larger gain differences between the WDM channels of approximately 10 dB are permitted.
Patent application EP 0139081 A2A describes a Raman amplifier having a number of Raman amplifier stages (RA1, RA2 . . . ) each having a number of pump sources whose wavelengths are close together. This makes the maximum of the Raman gain spectrum flatter, and the WDM channels at the amplifier output exhibit similar levels (page 13, lines 9–11).
EP 1 1018 666 A1 discloses a Raman amplifier having a number of pump sources whose signals have different wavelengths with intervals of 6 to 35 nm between them to even out the level curve of a WDM signal, so that it is not necessary to filter the levels using a “gain flattening filter”.
DE 199 10 041 A1 discloses an optical amplifier wherein a compensation pump signal is fed into a dispersion-compensating fiber, the amplitude and frequency of the signal being selected such that a required adjustment of the levels of the wideband signal to be amplified is achieved.
U.S. Pat. No. 6,115,174 discloses an optical transmission system having a pump source whose pump signals have different wavelengths. The wavelengths and the power of the pump signals are selected and varied such that a change in the level response of an amplified wideband signal can be set and varied respectively.
U.S. Pat. No. 6,097,535 describes a method and an arrangement for optical amplification of a WDM signal having a number of channels for which wavelength-dependent variations in the levels and signal-to-noise ratios are minimized. To flatten the signal-to-noise ratio spectrum, a first optical filter is disposed between two cascaded amplifiers. To flatten the level spectrum, a second optical filter is connected at the output of the amplifiers. Using two optical filters is costly here.
An object of the present invention is, therefore, to specify an amplifier with optimum signal-to-noise ratio curves. In addition, the gain must be as equal as possible for all the WDM channels.